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Datakommunikasjon Høsten 2002
Forelesning nr 2, 19. augustChapter 2, Application Layer
ØvingsoppgaverIngen
ApplikasjonsprotokollerFTP – File Transfer protocolDNS – Domain Name SystemHTTP – HyperText Transfer protocolTelnet, RloginSNMP – Simple Network Management ProtocolSMTP - Simple Mail Transfer ProtocolPOP3 – Post Office Protocol IMAP – Internet Mail Access Protocol
Kommunikasjonslagene (referert til OSI)
Application
Presentation
Session
Ethernet
IPARP
ICMP
TCP UDP
Network
Transport
Data Link
Physical
Application
Transport
Network
Data LinkPPP
FTP HTTP
DNS
OSIInternet-TCP/IP
SMTP
Network applications: some jargon
Process: program running within a host.
within same host, two processes communicate using interprocess communication (defined by OS).
processes running in different hosts communicate with an application-layer protocol
user agent: software process, interfacing with user “above” and network “below”. implements
application-level protocol
Web: browser E-mail: mail reader streaming
audio/video: media player
Client-server paradigmTypical network app has two
pieces: client and serverapplicatio
ntransportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Client: initiates contact with server
(“speaks first”) typically requests service
from server, Web: client implemented in
browser; e-mail: in mail reader
request
reply
Server: provides requested service to client e.g., Web server sends requested Web
page, mail server delivers e-mail
Application-layer protocols (cont).
API: application programming interface
defines interface between application and transport layers
socket: Internet API two processes
communicate by sending data into socket, reading data out of socket
Q: how does a process “identify” the other process with which it wants to communicate? IP address of host
running other process “port number” - allows
receiving host to determine to which local process the message should be delivered
What transport service does an app need?Data loss some apps (e.g., audio)
can tolerate some loss other apps (e.g., file
transfer, telnet) require 100% reliable data transfer
Timing some apps (e.g.,
Internet telephony, interactive games) require low delay to be “effective”
Bandwidth some apps (e.g.,
multimedia) require minimum amount of bandwidth to be “effective”
other apps (“elastic apps”) make use of whatever bandwidth they get
Transport service requirements of common apps
Application
file transfere-mail
Web documentsreal-time audio/video
stored audio/videointeractive games
financial apps
Data loss
no lossno lossloss-tolerantloss-tolerant
loss-tolerantloss-tolerantno loss
Bandwidth
elasticelasticelasticaudio: 5Kb-1Mbvideo:10Kb-5Mbsame as above few Kbps upelastic
Time Sensitive
nononoyes, 100’s msec
yes, few secsyes, 100’s msecyes and no
Internet transport protocols services
TCP service: connection-oriented: setup
required between client, server
reliable transport between sending and receiving process
flow control: sender won’t overwhelm receiver
congestion control: throttle sender when network overloaded
does not providing: timing, minimum bandwidth guarantees
UDP service: unreliable data transfer
between sending and receiving process
does not provide: connection setup, reliability, flow control, congestion control, timing, or bandwidth guarantee
Internet apps: application, transport protocols
Application
e-mailremote terminal access
Web file transfer
streaming multimedia
remote file serverInternet telephony
Applicationlayer protocol
smtp [RFC 821]telnet [RFC 854]http [RFC 2068]ftp [RFC 959]proprietary(e.g. RealNetworks)NSFproprietary(e.g., Vocaltec)
Underlyingtransport protocol
TCPTCPTCPTCPTCP or UDP
TCP or UDPtypically UDP
ftp: the file transfer protocol
transfer file to/from remote host client/server model
client: side that initiates transfer (either to/from remote)
server: remote host ftp: RFC 959 ftp server: port 21
file transfer FTPserver
FTPuser
interface
FTPclient
local filesystem
remote filesystem
user at host
ftp: separate control, data connections
ftp client contacts ftp server at port 21, specifying TCP as transport protocol
two parallel TCP connections opened: control: exchange
commands, responses between client, server.
“out of band control” data: file data to/from
server ftp server maintains “state”:
current directory, earlier authentication
FTPclient
FTPserver
TCP control connection
port 21
TCP data connectionport 20
ftp commands, responses
Sample commands: sent as ASCII text over
control channel USER username PASS password LIST return list of file in
current directory RETR filename retrieves
(gets) file STOR filename stores
(puts) file onto remote host
Sample return codes status code and phrase (as
in http) 331 Username OK,
password required 125 data connection
already open; transfer starting
425 Can’t open data connection
452 Error writing file
DNS: Domain Name System
People: many identifiers: SSN, name, passport #
Internet hosts, routers: IP address (32 bit) -
used for addressing datagrams
“name”, e.g., gaia.cs.umass.edu - used by humans
Q: map between IP addresses and name ?
Domain Name System: distributed database
implemented in hierarchy of many name servers
application-layer protocol host, routers, name servers to communicate to resolve names (address/name translation) note: core Internet
function, implemented as application-layer protocol
complexity at network’s “edge”
DNS - Domain Name System
Mapper mellom hostnavn og IP-adresse(og omvendt)
Benyttes av TCP/IP applikasjoner Distribuert, hierarkiskBenytter både TCP og UDP som
transport, port nummer 53 Eksempler
DNS Query DNS Reply
RFC1034, RFC1035
DNS name servers
no server has all name-to-IP address mappings
local name servers: each ISP, company has
local (default) name server host DNS query first goes
to local name server
authoritative name server: for a host: stores that
host’s IP address, name can perform name/address
translation for that host’s name
Why not centralize DNS? single point of failure traffic volume distant centralized
database maintenance
doesn’t scale!
DNS: Root name servers contacted by local name server that can not resolve name root name server:
contacts authoritative name server if name mapping not known
gets mapping returns mapping to local name server
b USC-ISI Marina del Rey, CAl ICANN Marina del Rey, CA
e NASA Mt View, CAf Internet Software C. Palo Alto, CA
i NORDUnet Stockholm
k RIPE London
m WIDE Tokyo
a NSI Herndon, VAc PSInet Herndon, VAd U Maryland College Park, MDg DISA Vienna, VAh ARL Aberdeen, MDj NSI (TBD) Herndon, VA
13 root name servers worldwide
Simple DNS example
host surf.eurecom.fr wants IP address of gaia.cs.umass.edu
1. contacts its local DNS server, dns.eurecom.fr
2. dns.eurecom.fr contacts root name server, if necessary
3. root name server contacts authoritative name server, dns.umass.edu, if necessary requesting host
surf.eurecom.frgaia.cs.umass.edu
root name server
authorititive name serverdns.umass.edu
local name serverdns.eurecom.fr
1
23
4
5
6
DNS example
Root name server: may not know
authoritative name server
may know intermediate name server: who to contact to find authoritative name server
requesting hostsurf.eurecom.fr
gaia.cs.umass.edu
root name server
local name serverdns.eurecom.fr
1
23
4 5
6
authoritative name serverdns.cs.umass.edu
intermediate name serverdns.umass.edu
7
8
DNS: iterated queries
recursive query: puts burden of
name resolution on contacted name server
heavy load?
iterated query: contacted server
replies with name of server to contact
“I don’t know this name, but ask this server”
requesting hostsurf.eurecom.fr
gaia.cs.umass.edu
root name server
local name serverdns.eurecom.fr
1
23
4
5 6
authoritative name serverdns.cs.umass.edu
intermediate name serverdns.umass.edu
7
8
iterated query
DNS: caching and updating records
once (any) name server learns mapping, it caches mapping cache entries timeout (disappear) after
some time update/notify mechanisms under design by
IETF RFC 2136 http://www.ietf.org/html.charters/dnsind-charter.html
DNS resource records
DNS: distributed db storing resource records (RR)
Type=NS name is domain (e.g.
foo.com) value is IP address of
authoritative name server for this domain
RR format: (name, value, type,ttl)
Type=A name is hostname value is IP address
Type=CNAME name is alias name for some
“cannonical” (the real) name
www.ibm.com is really servereast.backup2.ibm.com value is cannonical name
Type=MX value is name of mailserver
associated with name
DNS protocol, messagesDNS protocol : query and reply messages, both with same message format
msg header identification: 16 bit #
for query, reply to query uses same #
flags: query or reply recursion desired recursion available reply is authoritative
DNS - Domain Name System RFC1034, RFC1035
Distribuert Ingen navneserver har lagret all informasjon Et nett (firma, organisasjon o.l) har en eller flere
navneservere Inneholder hele eller deler av egne definisjoner Håndterer også forespørsler utenfra
Hierarkisk Hvis egen server ikke har nødvendig informasjon,
sendes forespørselen til nivået over Et overliggende nivå vil gjenkjenne nok til å kunne
velge underliggende nivå for forespørsel.
DNS - Domain Name System RFC1034, RFC1035
Top Level Domains
Second Level Domains
Unnamed root
IN-ADDR
36
136
69
196
YAHOO
PEOPLE
NO
SCANDPOWER
WWW
Generic Domains Country Domains
ARPA - Special Domain for address-to-name mappings
COM EDU GOV MIL NET ORGARPA AE NO ZW
DNS - Domain Name SystemRFC1034, RFC1035
Resultat fra en ekstern forespørsel kan lagres i lokal navneserver til senere bruk
En DNS respons vil inneholde informasjon om kilden er autoritativ eller ikke.
The Web: the http protocol
http: hypertext transfer protocol
Web’s application layer protocol
client/server model client: browser that
requests, receives, “displays” Web objects
server: Web server sends objects in response to requests
http1.0: RFC 1945 http1.1: RFC 2068
PC runningExplorer
Server running
NCSA Webserver
Mac runningNavigator
http request
http re
quest
http response
http re
sponse
The http protocol: morehttp: TCP transport
service: client initiates TCP
connection (creates socket) to server, port 80
server accepts TCP connection from client
http messages (application-layer protocol messages) exchanged between browser (http client) and Web server (http server)
TCP connection closed
http is “stateless” server maintains no
information about past client requests
http example
Suppose user enters URL www.someSchool.edu/someDepartment/home.index
1a. http client initiates TCP connection to http server (process) at www.someSchool.edu. Port 80 is default for http server.
2. http client sends http request message (containing URL) into TCP connection socket
1b. http server at host www.someSchool.edu waiting for TCP connection at port 80. “accepts” connection, notifying client
3. http server receives request message, forms response message containing requested object (someDepartment/home.index), sends message into socket
time
(contains text, references to 10
jpeg images)
http example (cont.)
5. http client receives response message containing html file, displays html. Parsing html file, finds 10 referenced jpeg objects
6. Steps 1-5 repeated for each of 10 jpeg objects
4. http server closes TCP connection.
time
Non-persistent, persistent connections
Non-persistent http/1.0: server parses
request, responds, closes TCP connection
each transfer suffers from TCP’s initially slow sending rate
many browsers open multiple parallel connections
Persistent default for htp/1.1 on same TCP connection:
server, parses request, responds, parses new request,..
client sends requests for all referenced objects as soon as it receives base HTML.
fewer RTTs, less slow start.
http message format: request two types of http messages: request, response http request message:
ASCII (human-readable format)
GET /somedir/page.html HTTP/1.0 User-agent: Mozilla/4.0 Accept: text/html, image/gif,image/jpeg Accept-language:fr
(extra carriage return, line feed)
request line(GET, POST,
HEAD commands)
header lines
Carriage return, line feed
indicates end of message
http request message: general format
http message format: response
HTTP/1.0 200 OK Date: Thu, 06 Aug 1998 12:00:15 GMT Server: Apache/1.3.0 (Unix) Last-Modified: Mon, 22 Jun 1998 …... Content-Length: 6821 Content-Type: text/html data data data data data ...
status line(protocol
status codestatus phrase)
header lines
data, e.g., requestedhtml file
http response status codes
200 OK request succeeded, requested object later in this
message
301 Moved Permanently requested object moved, new location specified later
in this message (Location:)
400 Bad Request request message not understood by server
404 Not Found requested document not found on this server
505 HTTP Version Not Supported
In first line in server->client response message.A few sample codes:
Cookies: keeping “state”
server-generated # , server-remembered #, later used for: authentication remembering user
preferences, previous choices
server sends “cookie” to client in response msgSet-cookie: 1678453
client presents cookie in later requestscookie: 1678453
client server
usual http request msgusual http response
+Set-cookie: #
usual http request msg
cookie: #usual http response
msg
usual http request msg
cookie: #usual http response msg
cookie-specificaction
cookie-specificaction
Web Caches (proxy server)
user sets browser: Web accesses via web cache
client sends all http requests to web cache object in web cache:
web cache returns object
else web cache requests object from origin server, then returns object to client
Goal: satisfy client request without involving origin server
client
Proxyserver
client
http request
http re
quest
http response
http re
sponse
http request
http response
origin server
origin server
Why Web Caching?
Assume: cache is “close” to client (e.g., in same network)
smaller response time: cache “closer” to client
decrease traffic to distant servers link out of
institutional/local ISP network often bottleneck
originservers
public Internet
institutionalnetwork 10 Mbps LAN
1.5 Mbps access link
institutionalcache
HTTP eksempel (GET)
(REQUEST METODE) Line 1: GET / HTTP/1.1
(REQUEST HEADER PARAMETER)Line 2: Accept: image/gif, image/x-xbitmap, image/jpeg, image/pjpeg, application/vnd.ms-excel, application/msword, application/vnd.ms-powerpoint, */* Line 3: Accept-Language: en Line 4: Accept-Encoding: gzip, deflateLine 5: If-Modified-Since: Wed, 26 Sep 2001 09:30:23 GMTLine 6: If-None-Match: "502728de6d46c11:1c8e”Line 7: User-Agent: Mozilla/4.0 (compatible; MSIE 5.5; Windows NT 4.0; TUCOWS)Line 8: Host: intranett.halden.scandpower.no
(GENERAL HEADER FIELD)Line 9: Connection: Keep-Alive
HTTP eksempel (Get response)
Line 1: HTTP/1.1 304 Not Modified
Line 2: Server: Microsoft-IIS/4.0
Line 3: Date: Sun, 04 Nov 2001 16:20:09 GMT
Line 4: Content-Location: http://intranett.halden.scandpower.no/Default.htm
Line 5: ETag: "502728de6d46c11:1c8e"
Line 6: Content-Length: 0
HTTP eksempel (response)
Line 1: HTTP/1.1 200 OK
Line 2: Server: Microsoft-IIS/4.0
Line 3: Date: Sun, 04 Nov 2001 16:20:09 GMT
Line 4: Content-Type: application/x-javascript
Line 5: Accept-Ranges: bytes
Line 6: Last-Modified: Fri, 02 Nov 2001 13:58:51 GMT
Line 7: ETag: "80f66b80a663c11:1c8e"
Line 8: Content-Length: 14481
Telnet og RloginInnlogging fra en maskin til en annen over
nettetBenytter seg av klient-tjener begrepetTelnet er en standard applikasjon som er
implementert i alle TCP/IP applikasjonerRlogin kommer fra Berkley Unix og ble
utviklet for pålogging mellom to Unix systemer
Telnet er mer kompleks enn Rlogin
44
SNMP – Simple Network Management Protocol
Request
Response
Unsolicited trap
Manager Agent
Network ManagementStation
Network Management Protocol Managed Node
(Management Information)
45
SNMP protokollen
Manager Agent
GetRequest,GetNextRequest,SetRequest
GetResponse
Trap
Port 161
Port 162
46
SNMP innkapsling
LLC/MACheader
IPheader
UDPheader
SNMPmelding
LLC/MACtrailer
Data Link nivå Nettverks-nivå
Transport-nivå
Applikasjons-nivå
SNMP innkapsling:
47
SNMPv1 melding
En SNMPv1 melding består av 3 deler:
Versjonsnummer
Communitystring
En av de 5SNMP PDUene
Internet Mail User agent, dvs Outlook, Eudora, Pegasus osv Mail transfer Agent, dvs Microsoft Exchange,
Sendmail SMTP - Simple Mail Transfer Protocol
TCP/IP Kun sending av tekst
MIME - Multi-purpose Internet Mail Extension Sending av bilder, video osv
POP 3 - Post Office Protocol ver 3 IMAP - Internet Message Access Protocol MX-records (Mail Exchange records)
Del an DNS (Domain Name System)
SMTP Mail Flyt
User Agent (mail program)Lese og sende mailOpsjoner:
Videresending til andre Svarsfunksjon Filtrering av innkommende mail til ulike mail
bokser Signatur fil Adresslister, aliases
Mail Transfer Agent (MTA)Ansvarlig for å sende mailen gjennom
nettetBaseres på SMTP (Simple Mail Transfer
Protocol)SMTP er en enkel ASCII protokollBenytter TCP og port 25 for å opprette en
forbindelse mellom to MTA-er
Sammensetning av en mailEnvelopes
Brukes av Message Transfer Agent
Headers Brukes av User Agent
Body Innholdet i mailen (tekst og vedlegg)
Envelopes - eksempelReceived: from sara.halden.scandpower.no ([193.69.136.35]) by
Received: from sara.halden.scandpower.no ([193.69.136.35]) by janis.halden.scandpower.no with SMTP (Microsoft Exchange Internet Mail Service Version 5.5.2653.13)id RCM02KCM; Mon, 20 Aug 2001 12:41:19 +0200
Received: from fw.scandpower.no (mail.hrp.no [158.36.61.51])by sara.halden.scandpower.no (8.9.3/8.9.3) with SMTP id MAA12382for <[email protected]>; Mon, 20 Aug 2001 12:43:02 +0200
Received: from mail.hrp.no ([158.36.61.51]) by fw.scandpower.no via smtpd (for sara.halden.scandpower.com
[193.69.136.35]) with SMTP; 20 Aug 2001 10:44:06 UTReceived: from pcthorbjornb (pc-thorbjornb.hrp.no [158.36.44.62])
by mail.hrp.no (8.10.1/8.9.0) with SMTP id f7KAlXK14155;Mon, 20 Aug 2001 12:47:33 +0200 (METDST)
Header - eksempelMessage-ID:
<806CBC6BC8F7D4118BB0009027A3176604EDC2@EXCHANGE>
From: Per Hansen <[email protected]>To: [email protected] <
[email protected]>Subject: E-mail security
SMTP-kommandoer (RFC 821)HELO <sending-host>MAIL FROM: <from-address>RCPT TO: <to-address>DATA<tekst>.QUIT
MX-recordsBrukes for å fortelle omverdenen om
hvem som er mail serverDel av DNS (Domain Name System)MX-recorden for en domene forteller i
prioritert rekkefølge hvor mailen skal sendes
MX-record eksempelMX-record for scandpower.no
1. prioritet: bill.halden.scandpower.no2. prioritet: mail.globalone.no
Mail leveres til mail.globalone.no hvis mailserver bill er nede eller forbindelsen til Internett er nede
POP 3Post Office Protocol number 3Protokoll for å hente mail fra mail server
til en mail klient (f.eks Outlook eller Eudora)
Bruker TCP og port 110Baserer seg på enkle ASCII kommandoer
POP3 kommandoerUSER usernamePASS passwordSTAT [gir antall uleste meldinger]LIST (n) 8gir størrelse på melding n]RETR n [hent melding nr n]DELE n [slett melding nr n]QUIT
Internet Message Access Protocol - IMAP Mail klientTilsvarende som POP3, men all behandling
av mail foregår på mailserverenPOP3 henter mailen ned til User Agent
MIME – Multipurpose Internet Mail ExtensionUtvidelse av SMTP for å kunne overføre
filer som ikke er 7-bit ASCIIMIME informasjon i mail:
MIME-Version Content-Type Content-Transfer-Encoding (Content-Description) (Content-ID)
MIME – Content TypeTextImageAudioApplication (Word, Postscript, )Multipart (Mixed, alternative)
MIME – Content-transfer encodingForteller hvordan innholdet av mailen er
kodet Fem forskjellige kode formater er definert
7 bits ASCII Quoted Printable base64 8 bits som inneholder linjer binær koding, 8 bit data uten linjer
Quoted Printable7 bit ASCII med alle karakterer <33 og
>127 kodes som likhetstegn + verdien av tegnet som to hexadecimale tegn
eks. bokstaven ”å” kodes som =E5Karakteresettet ISO-8859 gir å=229
desimalt229=11100101
1110=E1110=5”å” kodes som =E5
Base 64 Encoding
Base 64 encoding Tre bytes med data kodes som fire 6 bits
karakterer Orginale data: Hi!
H i !01001000 01101001 00100001 (24 bit)
010010 000110 100100 100001
00010010 00000110 00100100 00100001
S G k h
Datamengden øker med 25%
MIME - eksempelMIME-Version: 1.0X-Mailer: Internet Mail Service
(5.5.2653.19)Content-Type: text/plain; charset="iso-8859-1"Content-Transfer-Encoding: quoted-
printable
MIME eks. Word fil som vedlegg
MIME-Version: 1.0
Content-Type: multipart/mixed;
Content-Type: text/plain; charset="iso-8859-1"
Content-Type: application/msword;
Content-Transfer-Encoding: base64